Re: [CAD_CAM_EDM_DRO] stepper controller - help

----- Original Message -----
From: <JanRwl@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Sunday, November 17, 2002 5:23 AM
Subject: Re: [CAD_CAM_EDM_DRO] stepper controller - help


> In a message dated 11/16/2002 8:36:48 AM Central Standard Time,
> dmauch@... writes:
>
> > Not quite correct. The 65V is the maximum voltage that the manufacture
> > recommend those motors to run at. Those motors have a per phase
resistance
> > of 1.52 ohm so the actual voltage would be about 3.5V. So I would
recommend
> > a 36VDC power supply up to 65V maximum. But if you use the L298 then
you'd
> > better stick with the 35VDC power supply.
>
> Dan: For quite a time, now, I have been slightly frustrated that 98% of
the
> members of our group are more "machinists" than "E.E.'s", so they clearly
> have NO clue how a stepper with windings rated at, say, 4 amps, 3 volts,
need
> a 24 to 80 VDC supply. The concepts of "current-limiting",
voltage-dropping
> either via the nasty huge "L-R resistors" of the old "six-wire unipolar
days"
> or the more recently-espoused "chopper drive" technique float way above
most
> of our heads.
>
> Now I readily admit I am too ignernt to explain all this adequately in few
> words, but I am CERTAIN some E.E.'s in here like you, Mariss, etc., can do
so
> admirably. Thus, it seems a short "tutorial" on what L-reactance does
when
> stepping-rate exceeds three or four steps per second, and how modern
drives
> "deal with that" (and why 24 VDC or more is needed in a supply for "four
volt
> windings", etc.) would be very, very useful to us "in here"!!! Am I
right,
> guys?
>
> Jan Rowland, old troll
>
Hi,
I'm new to this group, only just found it. I'm an electrical fitter by
trade, went on to do an associate diploma in Electrical Engineering, that
puts me somewhere between a machinist and a proper Degree Electrical
Engineer.

About ten years ago, I got involved in a project to automate our High
Voltage and High Current Test sets, and we were using stepper motors to
replace manual control. We bought a state of the art bi - level chopper
drive, and it was very sweet. But our budget didn't stretch to 10 of them,
and it had lots of bells and whistles we didn't really need. So, we reverse
engineered the main circuitry, substituting parts which we could readily get
from Radio Spares or Farnell catalogue. We practically threw away most of
the design and had to start from scratch, using FETs on the outputs. Getting
it right got a bit exciting, particularly getting the bi - level bit
working. We got both drivers on at once one afternoon, and one of the output
transistors exploded, sending molten bits everywhere, burnt a hole clear
through my jumper at chest level. Sure is a great way to learn. Anyway, we
got it working, and started installing the first one on a machine just
before Christmas. During the Christmas holiday break, the skeleton crew, one
of whom was my foreman, and who were 'old trolls' so to speak, decided all
this new fangled electronics was not for them, and they rebuilt all the
manual controls using shafting, and bike chain and cogs.

Every cloud has a silver lining though, as all of the stepper motors, the
bi-level drivers, and bits and pieces that were now not allowed to be used,
well they must be surplus? I bought them at scrap value!

Enough gibberish, at the time I did manage to get the way the chopper drive
worked into my head, and I been dredging through the memory banks since I
read your post, trying to recall it. Maybe I can start the ball rolling on a
tutorial. Real Engineers feel free to bite me if I get it wrong, or anyone
else for that matter! Apologies if I go too simple or tell you what you
already know..

I'll try to answer some of your questions. I've found that just about every
electrical property or reaction has a mechanical equivalent, so I'll try to
throw in a mechanical analogy. Anyone who can get their head around
mechanical problems can do it with electrical too, if they can get past the
jargon.

Why does a stepper motor rated at 4 amps, 3 volts need 24 to 80 volts?

It's the inductive reactance of the circuit that is the culprit. The
magnetic field set up in the iron of stepper motor is like my work
colleagues, it doesn't like to change. An analogy that helped me understand
this was to think of a spinning bike wheel. If you have it out of the bike,
and grab it by both ends of the axle, and get someone to spin it, it's quite
happy. Try to twist it about on its axis, and it's gyroscopic effect fights
you. Inductive reactance is like that, try to change the current and it
resulting magnetic field, and it will actively fight you. That's why its
called reactance, it reacts to change.

Another name for voltage is electro motive force (EMF). It's this EMF we
apply to change the current through the stepper motor winding, think of it
as the force you apply to the bike wheel axle to try and move it. The bigger
the force, the faster you can move it. In the case of the stepper motor,
when you try to change the current using an EMF, the inductive reactance
generates an opposing EMF to fight the change, a 'back EMF'. How fast the
change happens depends on how strong your EMF is compared to the back EMF.

Why does the rpm of the motor come into play?
If you only had to set up the field once, then leave it be, things would
remain simple, and stepper motors would make great door stops.
Unfortunately, to get them to do anything useful, things have to keep
changing, and it's the rate of change that is important. Back, back you
demon calculus!
The inductance of the circuit is set by it's physical construction, and
doesn't change unless a physical change happens. In a bike wheel, where the
mass is in the wheel doesn't change either, well not under normal operation
anyway.

What does change with a change in rpm is the inductive reactance. Inductive
reactance equals twice times PI times frequency times the inductance. Since
the number two, PI, and inductance don't change, then changing the frequency
directly changes the inductive reactance. How hard the bike wheel fights
back is directly related to how fast you spin it. So if you spin the bike
wheel faster, it's harder to move. If you try to spin a stepper motor
faster, you have to overcome its reluctance to change as well, and one way
to do that is belt it between the ears with a relatively big EMF, say 50
volts.

The problem with the brute force technique is that it gets out of hand. You
still only want 4 amps to flow, but you want it to flow now! The big EMF
will get it going fast, and it will keep pushing up the current till it can
push no more, usually way above what the motor winding can handle. This is
when the smoke comes out. I've discovered that most electrical stuff runs on
smoke, if you let it out, they don't work any more. It's so hard trying to
get it back in there!

If you have ever been to a hands on science demo put on by some of the
museums, you might have seen the spinning bike wheel, and a platform that
can rotate, for you to stand on while you're holding the wheel. Imagine you
want to rotate around on this platform, slowly enough so that you don't get
too giddy. But you want to get from stationary to that rotation speed really
quickly. I don't know about you, but I just love acceleration. So you make
sure none of the museum staff are watching, and you get your friend to whack
the tire sideways with a big hammer. You sure get up to speed fast, but the
big hammer blow keeps on accelerating you, you end up going way too fast,
and fall off the platform.

What would be really handy, would be if the big hammer started it moving,
then as you got to the speed you wanted, it magically changed to a smaller
one, just big enough to keep you going. That is what the bi-level driver
does, it belts the stepper motor with a big EMF, it senses the current
rising, and as soon as it gets to 4 Amps, it switches the big EMF off and
very quickly turns on a smaller one, say 3 volts, to keep it happy.

I'm getting carried away here, and I haven't even talked about capacitors,
capacitive reactance, or resonance. Resonance is the reason I wouldn't use
stepper motors in open loop on anything that was doing serious work, and its
probably why DC servo motors are used on most commercial cnc machines.

Hope this has helped. Perhaps someone else would like to jump in and
continue?

regards,

John
from down under.